There is conventionally known a vehicle HUD device that displays a display light image, which is projected from a projector and reflected from a reflecting mirror, as a virtual image. When the reflecting mirror is used in this manner, the installation space required for the HUD device mounted in a vehicle can be reduced.
A vehicle HUD device disclosed in Patent Literature 1 is configured so that the rotation of a steeping motor is outputted in order to adjust the optical position of a reflecting mirror, and that the outputted rotation speed is reduced by a speed reducer gear mechanism and then transmitted to the reflecting mirror. More specifically, in the vehicle HUD device described in Patent Literature 1, a display range for enabling a user of the vehicle to view the virtual image and a reset range for inhibiting the vehicle user from viewing the virtual image are set as the mutually continuous ranges within which the stepping motor adjusts the optical position of the reflecting mirror. Within the reset range, a stopper mechanism disposed in the speed reducer mechanism stops the rotation of the reflecting mirror at an end opposite the display range.
Meanwhile, there is a concern in a typical vehicle HUD device that a rotary element may be displaced by vehicle vibration or other disturbance in a rotation transmission path between the stepping motor and the reflecting mirror.
In view of the above circumstances, the inventor of the present disclosure has conducted intensive studies to enable a stepping motor in a vehicle HUD device described, for instance, in Patent Literature 1 to output a rotation in a return-to-zero direction for the purpose of detecting a zero position at an end of the reset range that is opposite the display range. When the zero position is detected in the above manner, the optical position of the reflecting mirror can be accurately adjusted to an initial position based on the zero position within the display range by, after the detection of the zero position, causing the stepping motor to output a rotation oriented in an away-from-zero direction, which is a direction opposite the return-to-zero direction. Further, after the adjustment of the initial position, the optical position of the reflecting mirror can be accurately adjusted to a command position based on the zero position within the display range by causing the stepping motor to output a rotation responsive to a command from the vehicle user.
However, in order to detect the zero position and adjust the initial position, it is necessary to change the direction of rotation within the reset range and then change the optical position of the reflecting mirror into the display range. Therefore, a period of time required to determine the initial position is long. Consequently, an uncomfortable feeling may be given to the vehicle user. Further, the zero position is detected while the optical position of the reflecting mirror is held by the stopper mechanism. Therefore, when the stepping motor steps out, gears in the speed reducer mechanism may become rough to emit an obnoxious sound, giving an uncomfortable feeling to the vehicle user.